Let X₁, X_{2, ...,} X₁₀ be a sample from a N(25, 5²) distribution, independent of the sample Y_1, Y_{2, ...,} Y₂₀ coming from a N(0, 10²) distribution.

Standardize the following random variables:

a) X₁ - 25

b) (Y₁+Y₂+Y₃)/3